2-dradjat_hoedajanto

8/8/2019 2-dradjat_hoedajanto

1/6

EARTHQUAKE-RESISTANT DESIGN OF TALL BUILDINGS WITH BASEMENT AND

DEEP FOUNDATION ON SOFT SOIL

Dradjat Hoedajanto1)

and Awal Surono2)

Abstrack

The actual dynamic response of tall buildings on soft soil against earthquake loading is a very complicated one. The

desire to have in-elastic responses of the superstructures structural elements while maintaining the response of the

underground structural system remain elastic is not easy to model even with the current advances achieved in

available non-linear dynamic structural analysis commercial software. Thus, in this study a parametric study of

dynamic behavior of tall structural tower on soft soil are presented which used four type of soft soils configuration

representing the condition in Jakarta and Surabaya. The soil structure interaction option using elastic vertical and

lateral spring constants were utilized to account for the dynamic contribution of the soft soils on the lateral resistant of

the deep foundation system. The result were used to derive the indication on what should be the problem encountered.

The conclusion were then used to formed necessary basic concept on the design of earthquake-resistant of tall

building with basement and deep foundation on soft soil. The use of available Non-Linear Soil-Structure Interaction

Software should be tempered with caution. At best the result will only give indication on the actual behavior of thestructure and depend heavily on the accuracy of the non-linear characteristic of the material and elements considered

and assumed for the analysis. While the result shows that ground fixity level provide conservative result for the

superstructure, the reverse is true for the deep piling foundation system. Large lateral deformation of the foundation

system were observed indicating the possible significant reduction to the actual load carrying capacity of the piles.

Mass dan rigidity of the basement seems to play an important part of the load transfer capability of the whole

structure and hence should be carefully review together with the architects.

Keywords:Tall building, soft soils, earthquake-resistant, performance based, level of fixity, lateral deformation of the

roof, lateral deformation of the foundation base, reduction of load carrying capacity of piles, practical

guidelines of design steps for practicing engineers.

Abstrak

Respon dinamik aktual dari bangunan tinggi pada tanah lunak terhadap beban gempa merupakan masalah yang rumit.

Keinginan untuk mendapatkan respon in-elastikdari bangunan atas sementara respon dari bangunan bawah tetap

elastik merupakan hal yang sulit untuk dimodelkan sekalipun menggunakan perangkat lunak canggih yang terbaru.

Dalam studi parametrik ini, perilaku dinamik dari struktur tower tinggi pada tanah lunak dipresentasikan dengan

menggunakan empat tipe dari tanah yang mewakili kondisi tanah di Jakarta dan Surabaya. Opsi interaksi soil-

structure digunakan dengan menggunakan kekakuan vertikal dan lateral yang dipakai untuk memperhitungkan

kontribusi dinamik pada tanah lunak di dalam ketahanan lateral dari sistim pondasi dalam. Hasil yang ada akan

digunakan untuk menunjukkan indikasi problem yang terjadi pada struktur. Kesimpulan perlu diambil untuk

mendapatkan konsep dasar yang perlu dalam merencanakan bangunan tinggi tanah gempa dengan basemen dan

pondasi dalam pada tanah lunak. Penggunaan dari non linear soil structure interaksi harus dipakai secara hati-hati.

Hasil maksimal hanya akan memberikan perilaku aktual dari struktur dan hal ini tergantung sekali terhadap

keakuratan dari karakteristik non-linier dari material dan elemen yang dipakai serta asumsi analisis. Hasil analisa

menunjukkan bahwa ground fixity level memberikan hasil yang konservatif pada struktur atas, kebalikannya benaruntuk sistem pondasi dalam. Deformasi lateral yang besar pada sistem pondasi yang diamati memberikan hasil

kemungkinan reduksi dari gaya aktual pada pondasi. Massa dan kekakuan dari basementsepertinya menunjukkan hal

yang penting dalam mentransfer gaya ke seluruh struktur dan harus direncanakan secara hati-hati bersama-sama

dengan arsitek.

1 Assoc. Prof. Civil Engineering Department, Faculty of Civil Engineering and Environment, Institut Teknologi Bandung,Bandung, Indonesia

2 Assist. Prof. Civil Engineering Department, Faculty of Civil Engineering and Environment, Institut Teknologi Bandung,Bandung, Indonesia


2/6

Kata kunci : Bangunan tinggi, ketahanan gempa, performance based, level of fixity, deformasi lateral pada atap,

deformasi lateral pada pondasi, reduksi dari gaya terhadap kapasitas tiang,practical guidelines of design

steps for practicing engineers

1. INTRODUCTION

The actual dynamic response of tall buildings on soft soil against earthquake loading is a very

complicated one. The desire to have in-elastic responses of the superstructures structural elements

while maintaining the response of the underground structural system remain elastic is not easy to

model even with the current advances achieved in available non-linear dynamic structural analysis

commercial software.

For structural engineers, the common available design practices started with the assumption that the

fixity level of the superstructure maybe assumed at ground level. Design of the superstructure was

then carried out using the standard procedure of strength design method for live safety design concept

of earthquake-resistant building. Design of the basement and the supporting foundation system were

then carried out by applying the adjusted load from the superstructure assuming that the system

should remain elastic to exclude the possibility of excessive structural damages on below groundstructural elements. In the case of soft supporting soils, one of the most common questions pop up in

the structural engineers mind is whether it is still safe to treat the basement and deep foundation

separately from the total structural system and each element may still be designed accordingly.

The uncertainty on the most appropriate procedure of how the total building should be modeled and

designed started indirectly from the fact that most references available on the subject (Fintel, 1985;

Khan, 1985; Derecho et al., 1985; Paulay et al., 1992; Naeim, 2001; Filippou et al., 2004; Bertero et

al., 2004; Garcia et al., 1997) assumed that the example buildings are fixed at ground level. The non-

elastic nature of the response of the supporting soft soil interaction against the deep foundation

elements, the case where significant depth of the upper part of the supporting soils is soils with

average N-SPT value


3/6

rule may define and accepted as the one and only criteria to the problem. Proper judgments, guided

by seasoned related experiences, should rule.

2. CASE STUDIEDIn the following the three questions raised earlier will be conceptually discussed and reviewed.

2.1 Level of Fixity of SuperstructureSurono and Hoedajanto (Surono et al., 2006) show that the degree of softness of the supporting soil

does significantly alter the position of the possible fixity level of the tall superstructure on soft soil

under earthquake loading, see Figure 1. Consequently the value of fundamental period of the building

and the base shear at ground level will also be different as shown in Table 1 below.

Figure 1. Technical Description of the Case Studied

Table 1. Result of Analysis Assuming Linear Spring Constant for Lateral Soil Resistant

Max Internal forces on top of

pile

Soil

Type

Fundamental

Period (s)

M

(kN.m.)

V

(kN)

N

(kN)

Possible

level of

fixity (m)

found(mm)

roof(mm)

Depth of

very soft

top soil

(m)

1 3.21 139 91.3 675 - 5 47.5 244 10

2 3.31 133 75.0 658 - 8 62.4 254 15

3 4.87 130 25.6 637 - 9 180 246 154 8.50 130 19.2 552 - 24 436 150 15

Analysis of the superstructure alone (fixity at ground level) give Fundamental Period T of about 1.38

sec., Base Shear V = 4470 kN (about 93 kN per pile), M = 173000 kN.m, N = 12000 kN, and roof =

270 mm. Upon applying the external forces to the foundation, found for case 1, 2, 3, and 4 are 23.3

mm, 36.6 mm, 123 mm, and 360 mm respectively.


4/6

The case studied showed that:

1. Except for the value of Shear force V acting on the foundation (represent the value of base shearacting on the superstructure), the bending moment and the axial force for all cases considered do

not differ too much,

2. The assumed fixity at base gives conservative result for the case of Base Shear V and Total(Roof) Drift roof. While technically this conservative result looks better from the design point of

view, it should be carefully considered if consequently it leads to excessive construction cost,

3. Considering the result of the foundation lateral deformation, which represents the possibleunfavorable condition for the total foundation system, care should be taken to ensure that no

failure of the piling system may occur due to excessive lateral movement of the upper part of the

pile. The design of the pile should consider the effect of P- action which may leads to much

lower useable axial capacity of the pile.

2.2 Limit of Lateral Roof DeformationThe roof lateral deformation directly represents the level of damage experienced by the building

particularly the upper structure. This is an important issue for the case of Performance Based Design

where building damages become the main consideration for design target performance. Result of the

analysis shows that the common assumption of fixity at ground level leads to a conservative design

and hence should be welcome. Caution however should be raised since it underestimates the lateraldeformation for the foundation system. The result may become worse if instead of elastic spring

constant an in-elastic spring characteristic were used for the analysis. The case need further detail

studies and investigations.

2.3 The Use of Available Non-Linear Soil-Structure Interaction Software

While it is true that the advancement achieved in several commercial structural design software is

outstanding, the fact remain that at best they are accurate only within the basic assumptions set fornon-linear behavior of the materials and elements considered. For the case of the response of soft soil

due to ground shaking experiences during earthquake loading, the level of strain experienced during

the whole loading dictate the cyclic behavior the response. To date the full understanding of such case

is still under study (Irsyam). Limited insight to the case maybe obtained from Chapter 19 of ASCE 7-05 Standard (ASCE 7-05, 2006) on Soil Structure Interaction for Seismic Design, pp. 201 - 203.

Considering all the above, the best way a practicing engineer can do is to perform both elastic and in-

elastic analysis and try to physically understand the ramification of the whole process of the analysis,

especially on setting the assumption for the material properties of the structural elements, including

the spring characteristic of the supporting soils. Understanding that the basic concept of structural

design in this case is to provide a reasonably safe structure within the current state-of-the-arts of

design technology, bracketing the result of the analysis seems to be a safe and reasonable way out.

Safety however should be carefully judge and measured to construction and maintenance (including

repair) cost. This is where the concept of Performance Based Design can be utilized to its optimum.

Target Performance should then be a matter of mutual agreement among all concern, most

importantly the owner and potential users of the building.

3. CONCLUSION AND RECOMMENDATIONBased on the above limited studies, the following conclusion and recommendation maybe submitted:

1. The assumption of fixity at base level in most cases give a conservative result for the upperstructure but may significantly under estimate the design requirement for the foundation system


5/6

as a whole. Since conceptually any kind of failure is not desirable for any part of the structure

below ground, it is imperative that a more detailed and thorough studies on this problem should

be carried out before a designer settle for his final detailed design. Safety, tempered with caution

on its cost implication, should be the prime motivation of his decision.

2. To ensure that good fixity may be obtained at the junction of the upper structure and the basementand foundation system, especially to ensure that enough mass (compared to the mass of the upper

structure) and rigidity are present at the basement part of the structure. Structural engineersshould help the architects to design the size and the structural system of the basement. One of the

important things to do is to decide the size and thickness of the base slabs. This is where the

initial transfer of the action from the upper structure to the foundation system happened. The

concept of maintaining all of the below ground structural elements remain elastic should be kept

in mind at all time.

3. The design of the vertical structural elements of the basement should consider the maximumforces experienced if the level of fixity is not at ground level. In this case the practical practice is

to consider the result of the analysis if the level of fixity at basement floor level. Hence if we

have 2 basement floor levels, we should also consider fixity at B1 and B2 level. Choose the

maximum internal forces to design the element considered.

4. Choosing the proper element and material for the deep foundation system is very important.Lateral rigidity of the pile foundation system should be considered and choosing a more rigid

system, in this case larger diameter of piles seems to be the better solution to the problem.

5. The calculation of the actual axial pile capacity should be carefully carried out considering theoverstrength capacity of the vertical components of the superstructure at ground level and the

possible maximum lateral deformation of the foundation system (P- effect).

6. Detailing of the upper section of the pile should consider confinement requirement set forcolumns on soft story cases.

7. This study should be used with caution and may not be considers as a final indication of whatmay happen to the similar cases. It is entirely possible that different structural configuration mayleads to significantly different results.

4. REFERENCESASCE 7-05 (2006) Minimum Design Loads for Buildings and Other Structures, ASCE Standard,

USA.

Bertero, R.D., and Bertero, V.V. (2004) Performance-Based Seismic Engineering: Development

and Application of a Comprehensive Conceptual Approach to the Design of Buildings.

Earthquake Engineering from Engineering Seismology to Performance-Based Engineering,Bozorgnia, Y., and Bertero, V.V., Editors, CRC Press, Florida, USA.

Derecho, A.T., Fintel, M., and Ghosh, S. K. (1985)Earthquake-Resistant Structures. Hand-book of

Concrete Engineering, Second Edition, Mark Fintel Editor, Van Nostrand Rein-hold Company,

New York.


6/6

Filippou, F.C., and Fenves, G.L. (2004) Method of Analysis for Earthquake-Resistant Structures.

Earthquake Engineering from Engineering Seismology to Performance-Based Engineering,

Bozorgnia, Y., and Bertero, V.V., Editors, CRC Press, Florida, USA.

Fintel, M. (1985)Multistory Structures. Handbook of Concrete Engineering, Second Edition, Mark

Fintel Editor, Van Nostrand Reinhold Company, New York.

Garcia, L.E., and Sozen, M.A. (2004) Earthquake-Resistant Design of Reinforced ConcreteBuildings. Earthquake Engineering from Engineering Seismology to Performance-Based

Engineering, Bozorgnia, Y., and Bertero, V.V., Editors, CRC Press, Florida, USA.

Hoedajanto, D. (2006) Critical Review of The New Indonesian Earthquake Code SNI 03-1726-2002.

Indonesian Society of Civil and Structural Engineers Annual Conference, Jakarta Indonesia

(in Indonesian).

ICBC (1997) Uniform Building Code, Volume 2, USA.

International Building Code (2006), International Code Council. Inc., USA.

Irsyam, M. Private communication. Associate Professor - Civil Engineering Department, InstituteTeknologi Bandung - Indonesia.

Khan, F. R. (1985) Tubular Structures for Tall Buildings. Handbook of Concrete Engineering,

Second Edition, Mark Fintel Editor, Van Nostrand Reinhold Company, New York.

Naeim, F. (2001) The Seismic Design Handbook, Kluwer Academic Publisher, Second Edition.

Paulay, T., Priestley, T. (1992) Seismic Design of Reinforced Concrete and Masonry Building, John

Wiley & Sons, Inc..

Surono, A., Hoedajanto, D. (2006) Parametric Study of Dynamic Behavior of Structural Tower on

Soft Soil. Indonesian Society of Civil and Structural Engineers Annual Conference, Jakarta -Indonesia (in Indonesian).

Documents

2-dradjat_hoedajanto